The present invention relates to a glass composition for a lens that can be molded by hot pressing of a glass preform between metal molds with curved lens surfaces, particularly to glass for mold lens having a high refractive index and low dispersion.
Conventionally, a composition of optical glass has been formulated to give necessary characteristics such as refractive index, Abbe""s number, and chemical durability, without taking into account lowering of the glass transition temperature (Tg) of the optical glass.
Hot press molding technologies for a mold lens were developed since around 1983. Hot press molding requires no polishing and is capable of accurate control of a surface shape and surface roughness equivalent to those of a polished surface.
Glass for such mold lens is used at present for a pickup lens of an optical disk apparatus.
A mold lens can be obtained by pressing for a long time (about 20 sec.) at a temperature 10 to 20xc2x0 C. higher than a flexure temperature of glass, using accurately ground and polished metal molds.
The metal mold surface is coated with a thin film of diamond-like carbon (DLC) or TiCN in order to release the mold lens from the metal molds.
The glass used for mold lens glass is required to be low in Tg for prolonging the life of the mold coating and reducing the production cost, while it is desirable for the glass to be high in refractive index to reduce aberration of lens such as spherical aberration. That is, mold lens glass is required to be low in Tg and high in refractive index (nd).
Further, in a case of glass that is easily crystallized, it is difficult to reliably produce an excellent lens, and therefore, a glass composition that hardly crystallizes is required.
Also, in a case of glass having poor water resistance, a surface of the glass reacts with moisture in the air, causing whitening of the glass surface, and therefore, the water resistance of glass has to be sufficiently high.
The present invention is intended to provide mold lens glass having excellent water resistance, low Tg and high refractive index (nd) and is hard to crystallize during a process of producing the glass.
The mold lens glass of the present invention includes B2O3, SiO2, La2O3, ZnO, BaO, and CaO. A glass composition includes 25% to 35% by weight of B2O2 and 1% to 7% by weight of SiO2. The composition includes 30% to 40% total weight of B2O3 and SiO2, 12% to 20% by weight of La2O3, and 40% to 55% by total weight of ZnO, BaO and CaO.